Analysis On The Relationship Between Oxidation Kinetics And Oxide Film Structure Evolution Of The Nickel-based Superalloy

For increasing the service performance of the turbine blades,such as the aero-engine power and thermal efficiency,the components were demanded for withstanding high temperatures.Under such conditions,the high-temperature oxidation will inevitably become a key factor for causing these components failure,which brings about a catastrophic consequence.Therefore,studying the high temperature oxidation behavior of directionally solidified nickel based superalloy,which worked as the key components of aeroengine,become critically important.At present,for one thing,the relationship between the oxidation mass gain kinetics and oxide film thickened kinetics need to be clarified;for another thing,the oxide film structure affected by many factors,such as surface signature,temperatures and alloying elements,however,the behind relationship between the oxide film structure and oxide film evolution among these different conditions need to be further revealed.In this paper,the oxidation kinetics and oxide film structure of DZ125 and DD6 directionally solidified nickel based superalloy,which produced in China,were investigated among the different conditions.The relationship between the oxidation mass gain kinetics and oxide film thickened kinetics were clarified.In addition,the oxide growth mechanism was revealed under different conditions.Moreover,the relationship between the oxide film structure and oxidation mass gain kinetics were established.The detailed contents are as following:(1)For the study on the oxidation kinetics of DD6 and DZ125 alloy:it was found that the mass gain kinetics and oxide film thickened kinetics exhibited a similar characteristic,which showed a good agreement with each other.When Ra=0.6?0.7 ?m,no staged phenomenon was observed in the oxidation kinetics of two alloys.Meanwhile,the oxidation reaction index n was approximately 0.2,which indicated that the oxidation kinetics obeyed an asymptotic horizontal line law.The staged phenomenon occurred when Ra=0?0.1?m and Ra=0.1?0.2 ?m as well as Ra=0.3?0.4 ?m.Moreover,the oxidation reaction index n displayed a large difference at the first stage and second stage of two alloys.At the first stage,the oxidation kinetics of DD6 alloy and DZ125 alloy obeyed cubic law(n?0.3)and parabolic law(n?0.5),respectively,whereas the second stage obeyed an asymptotic horizontal line law(n=0.1?0.2)of two alloys.(2)Two oxidation modes were proposed through the analyzing of the interaction between the oxide film structure and oxidation kinetics of two alloys:the behavior of the single-layer structure accompanied with the no-staged oxidation kinetics was called type I growth mode.While the multi-layer structure with the staged oxidation kinetics was called the type ? growth kinetics.It was revealed that the growth rate difference of various layers resulted in the oxidation mass gain kinetics appeared staged phenomenon.The growth of outer layer and inner layer were responsible for the weight gain of the first stage,while the intermediate layer determined the weight gain of the second stage.(3)For the study of the oxide film morphology and elements distribution as well as phases structure after short time and long time oxidation under the type I growth mode and type ? growth mode:the oxide film structure was mainly composed of Al2O3 of the two alloys under the type I growth mode.Under the type ? growth mode,the oxide film was made of multi-phases.The short time oxide film structure of DD6,from the top surface to the base matrix,was composed of NiO,Ni(Co)Cr2O4,Ta2O5 and discontinuous Al2O3.While DZ125 was made of Cr2O3,un-oxidized zone,Ta2O5 and discontinuous Al2O3.The long time oxide film structure of DD6,from the top surface to the base matrix,was composed of NiO,CoAl2O4,CoCr2O4,Ta2O5,NiAl2O4 and continuous Al2O3.While DZ125 was made of NiO,NiCr2O4,CoCr2O4,Ta2O5,NiAl2O4 and continuous Al2O3.(4)For analysis of the formation conditions of the oxide film products:the formation condition of initial oxide products was clarified.Al2O3 preferably formed at the high surface roughness,which was a necessary condition for the oxidation behavior of the alloy following the type I growth mode.Whereas the difference of Cr contents was a sufficient condition for the existence of a large difference in the initial oxide species of two alloys under the type II growth mode.In addition,the influence of surface roughness on the critical contents of external oxidation of Al2O3 in the nickel based superalloys was revealed.The promotion of short-circuit diffusion on a high-roughness surface causes the critical concentration line to gradually shift toward a low Al concentration direction,while the effect of temperature on the concentration critical line is relatively small.(5)For the study on the oxide film evolution behavior of two alloys:it was shown that under the type I growth mode,the oxide film distribution characteristics of DD6 alloy was globular Ta2O5 randomly doped at the middle position of the Al2O3 layer,while the DZ125 alloy was HfO2 randomly doped at the lower position of the Al2O3 layer.Under the type II growth mode,DZ125 alloy displayed that the exterior layer Cr2O3 was gradually covered by NiO with the increasing of oxidation time,meanwhile,the middle layer transformed from unoxidized area into a series of complex spinel phases.It was found that the evolution of the oxide film depending on the enrichment or depletion of elements in the ?'-free zone.The enrichment degree of Co and Cr solutes in the ?'-free zone under type II mode gradually increased with the extension of the oxidation time,which promoted the growth of the oxide film in the intermediate layer.(6)For the study on the oxide film growth mechanism of two alloys:it was found that the oxide film evolution and the distribution characteristic of the oxidation rate constant k showed good correspondence.Under the type I growth mode,the oxide film is composed of continuous dense Al2O3,while k distributed near the Al2O3 attachment.Similarly,the type II growth mode obeyed a similar pattern.The oxidation kinetics characteristics of the alloy are closely depending on the growth mechanism of oxide film,and have a direct relationship with the morphology of Al2O3.When the oxidation mechanism of the alloy is dominated by the diffusion of metal cations through Al2O3,the n was 0.1?0.2.When the discontinuous Al2O3 accompanied with loose outer layer was developed on the surface,the double diffusion of outwardly metal cations and inwardly oxygen anions were responsible for the oxidation mechanism,corroding to the n?0.3.When a discontinuous Al2O3 accompanied with a dense outer chromium layer established on the surface,the diffusion of metal cations through the outer layer controlled the oxide growth,and the oxidation reaction index is n?0.5.